Comparative study on secondary xylem and formation of successive cambia in stems and roots of Antigonon leptopus Hook. & Arn. (Polygonaceae)

Abstract

Antigonon leptopus Hook. & Arn. (Polygonaceae) being a climbing member increases their stem and root diameter by forming successive cambia. The first successive ring of cambium initiates as small segments from the parenchymatous cells located on the inner side of group of primary phloem fibres. These small arcs of cambia produced secondary xylem internally and phloem externally while portion between newly initiated cambial arcs exclusively produce thin walled parenchyma cells on either side. With the increase in age, vessel diameter and their frequency not only decreased in roots but also showed complete absence of wider vessels or observed occasionally in thick roots. In thick stems, strands of lignified derivatives once closely arranged became separated and appeared as embedded in parenchymatous ground mass. At this stage, these parenchyma cells underwent proliferation and formed small arcs of meristematic cells and produce vascular elements. Formation of xylem and phloem from these arcs appear like vascular bundles embedded in conjunctive tissue. Since, major portion of the stem is composed of parenchyma cells, presence of fibre with distinct bordered pits instead of wider vessels might be related with additional mechanical support and to provide additional path for the water conduction in absence of wider vessels in roots.